Alkali alumino-silicate glasses that contain magnesium oxide are being used in applications where mechanical performance is more important. These glasses can be formed using a fusion draw process, where liquid glass flows over the lips of a glass overflow forming block made of zircon material and fuses at the bottom of the glass overflow forming block to form a glass sheet. In contact with alkali alumino-silicate glasses, zircon (ZrSiO4) dissociates into ZrO2 and SiO2 at temperatures close to the glass forming temperature. The higher SiO2 content may lead to formation of gas bubbles as it dissolves into the glass. ZrO2 can create ZrO2 solid nodules at the interface that can then be released into the glass forming defects. Accordingly, the glass overflow forming block has a reduced lifetime as zircon material erodes from the body of the glass overflow forming block while the glass that is manufactured is contaminated with an undesired element that disadvantageously affects its properties.
The use of the same reference symbols in different drawings indicates similar or identical items.